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56 SMT007 MAGAZINE I MAY 2018 Precut Tape Dots Precut tape dots are effective for conformal coating masking of flat areas to the PCB areas like test points and mounting holes. These are precut stickers, usually round, of standard sizes and die cut; they are available in poly - ester, polyimide, Kapton [1] and other materi- als. They are supplied on a roll and are pulled off the release liner one at a time. They are cost-effective for small to medium production runs. Laser Cut Tape Shapes Laser cut tape shapes are precision cut from flat sheets of tape material and can be fabricated using Kapton, flexible tape like 3M Type 401 [2] or others. This method is ideal for complex shapes for prototype or small- volume builds as they reduce masking time and can be delivered quickly. The shapes are generated from the board's Gerber files and can be used not only to protect flat areas but can be designed to tape over 3D components like connectors and headers requiring mask- ing. The technician simply peels the mate- rial off the release liner and places it on the board or over the component. The shapes can be grouped together to visually indicate that each tape has been removed from the backing material, helping ensure that all areas requir- ing masking are masked. Hand Cut and Trimmed Tape In this masking method, the tape shapes are cut to size either coming right off the roll or the component is wrapped by the rework techni- cian and trimmed afterwards. Performing the cutting operation on the board is dangerous as parts or boards can be damaged. Perform- ing the cutting after peeling back the roll can be dangerous as the operator can potentially cut their hands. For prototypes or for very odd-shaped areas, it is a very flexible masking method. However, it is a very time-consuming process for both masking and de-masking. The charge generation process during de-masking operations must ensure that the components and assembly are protected from any adverse ESD events (sometimes requiring an air ionizer in the masking/de-masking area). Liquid Latex, aka "Peelable Mask" Liquid latex is a thixotropic material designed to be, among other things, a protection against unwanted conformal coating spray. It can be dispensed from a syringe or plastic-tipped squeeze bottle. It is typically cured on the board at room temper- ature over a three-hour range. After curing and application of the conformal coating, the mask- ing material is simply peeled from the board and disposed of with the regular waste stream. This is ideal for areas that may be tough to get at with the tape. In addition, this type of masking material is ideal if you do not want the conformal coating "wicking up" the contact leads. At times, both the taping methods and the peelable mask method will be employed on the same board with the peelable mask at the board interface. Be careful in using natural latex as these formu- lations are heavily ammoniated and therefore can impact the PCB lami- Figure 1: Pre-cut tape shapes for masking.

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